The long term objectives of this proposal are to understand the process of evolution in asexual microorganisms, and the genetic structure of their populations. Recent work in thsi area has revealed that asexual microorganism populations possess a level of genetic complexity far greater than previously supposed. The specific aims of this project will be to analyse the factors influencng the evolution of population stucture in the yeast Saccharomyces cerevisiae. We will focus on adaptive mutations arising in long-term laboratory populations. The great majority of the adaptive mutants have already been identified from earlier work, and samples containng them are maintained in storage at -70oC. In particular we plan a) to analyse the role of transposable elements and gross chromosomal rearrangements (including loss) in the generation of thes adaptive mutations, b) determine the fitness interactions between them, c) determine the origins and genealogical relationships of the adaptive mutants to understand how population structure changes an evolves and d) assess the development of genetic polymorphism and environmental complexity in simple laboratory populations where growth is limited initially by a single resource. For the most part we will use well developed and documented microbiological and molecular genetic techniques. As well as being important to population genetics an evolution, these studies will provide important information on the evolution and ageing of proliferating cell populations. The complete understanding of cell proliferation and the genetic changes that the cells undergo is critically important to many aspects of medicine, not the least of which are cancer treatment and ageing.